1. Field of the Invention
This invention relates to expandable endoprosthesis devices, generally called stents, which are adapted to be implanted into a patient""s body lumen, such as blood vessel, to maintain the patency thereof. These devices are very useful in the treatment of atherosclerotic stenosis in blood vessels. The invention more particularly relates to a method for increasing the stent retention properties of a stent delivery system.
2. Description of Related Art
Stents are generally tubular-shaped devices which function to hold open a segment of a blood vessel or other anatomical lumen. They are particularly suitable for use to support and hold back a dissected arterial lining which can occlude the fluid passageway therethrough.
Various means have been described to deliver and implant stents. One method frequently described for delivering a stent to a desired intraluminal location includes mounting the expandable stent on an expandable stent delivery member, such as a balloon, provided on the distal end of an intravascular catheter, advancing the catheter to the desired location within the patient""s body lumen, inflating the expandable stent delivery member on the catheter to expand the stent into a permanent expanded condition and then deflating the expandable stent delivery member and removing the catheter. One of the difficulties encountered using prior stent delivery systems involved retaining the stent on the expandable stent delivery member during delivery of the stent to the desired location within the patient""s body lumen, while at the same time maintaining the ability of the expandable stent delivery member to release the stent when desired, and maintaining sufficient longitudinal flexibility of the stent to facilitate its delivery.
Currently, stents are commonly mounted over a stent delivery balloon by placing a stent over a folded stent delivery balloon, placing the assembly in a tube or similar device to prevent the stent from expanding while the stent delivery balloon is pressurized, applying heat to the assembly, and pressurizing the stent delivery balloon. This causes the stent delivery balloon to expand somewhat between the struts of the stent, resulting in inclined areas of the stent delivery balloon to provide mechanical interference retaining the stent on the stent delivery balloon. A current trend is to design stents that are thinner and that have a smaller outer diameter, while still providing as much surface area as possible to provide scaffolding to a stenosis of a vessel. As a result, in more advanced thinner stent designs, the amount of mechanical interference provided by mounting of the stent on a stent delivery balloon in this manner is reduced. Also, once the stent moves out of the set position on the stent delivery balloon, retention forces decrease significantly, because the raised regions of the stent delivery balloon no longer match the pattern of the stent struts, and the mechanical interference of the raised, inclined areas of the stent delivery balloon is largely lost. The coefficient of friction of commonly used balloon materials such as nylons, polyethylene terephthalate, polyethylene, and the like, is typically not very high, and stent struts are typically polished or rounded, so that once the stent is displaced even slightly from the set position on the stent delivery balloon, the forces retaining the stent on the stent delivery balloon are greatly reduced.
One prior art stent delivery system provides for retention of a stent on a balloon by a capsule of material that slowly dissolves in blood. However, encapsulation of a stent can interfere with the release of the stent upon inflation of the balloon, can unacceptably increase the stent implantation procedure time due to the time required for the capsule of material to dissolve, and can reduce flexibility of the combination of the stent and the stent delivery balloon. The dissolving of an encapsulating material within the bloodstream can further release particles large enough to block blood flow downstream in the arterioles or capillary bed, and can contribute to thrombogenesis.
What has been needed and heretofore unavailable is a system and method for retaining a stent on an expandable stent delivery member that allows an assembly of a stent and a stent delivery system to maintain a high degree of flexibility, that allows the stent to be readily expanded and released when desired, and that does not utilize material that dissolves in the blood. The present invention addresses these and other needs.
Briefly, and in general terms, the present invention provides for retention of a stent on a stent delivery member without compromising the high degree of flexibility of the stent and stent delivery system needed to allow them to be advanced through the tortuous passageways of the vasculature, allowing the stent to be readily expanded and released when desired, allowing the stent to have the mechanical strength to hold open the body lumen into which it is expanded, and without contributing to thrombogenesis.
The present invention accordingly provides for a method for retaining a stent on an expandable stent delivery member, comprising the steps of applying a plastic material that is insoluble in blood to at least one of the expandable stent delivery member and the stent, and mounting the stent over the expandable stent delivery member. The plastic material will preferably adhere or fuse to the expandable stent delivery member. In another presently preferred aspect, the plastic material has a relatively high coefficient of friction. In a presently preferred embodiment, the expandable stent delivery member is an inflatable balloon member. The plastic material may be chosen from plastics, organic materials, resins, two part resins, polymers, copolymers, and combinations thereof. In a presently preferred embodiment the plastic material is chosen from ethylene acrylic acid (EAA) copolymers, polyvinyl chloride (PVC), ethylene vinyl acetate copolymer (EVA), ethylene glycol butyl ether acetate (EBA) and ethylene methyl acrylate (EMA) ethylene acrylic ester copolymers, ethylene acrylic ester maleic anhydride terpolymers, and acid copolymer resins. The plastic material may also incorporate a therapeutic agent, such as a compatible anti-thrombus agent, a drug for reducing the likelihood of clots forming on the stent and the stent delivery member during exposure of the stent and the stent delivery member to blood, an anti-restenosis agent, and combinations thereof. The plastic material may also advantageously include a cross-linking agent chosen from cross-linking catalysts and resins.
In order to improve the retention of the stent on the expandable stent delivery member, heat can be applied to the stent mounted over the expandable stent delivery member to enhance cross-linking of the plastic material. In an alternate approach, the step of mounting the stent over the expandable stent delivery member comprises disposing the stent on the expandable stent delivery member, and applying heat and pressure to the stent and the expandable stent delivery member to create an adhesive bond between the stent and the expandable stent delivery member. In a presently preferred embodiment, the plastic material will adhere to the expandable stent delivery member, and the surface of the expandable stent delivery member may also be etched prior to the step of applying the plastic material, to improve adherence of the plastic material to the expandable stent delivery member. Areas of the expandable stent delivery member may be masked prior to etching the surface of the expandable stent delivery member. In another aspect, a release agent may optionally be applied to one or both of the stent and the expandable stent delivery member prior to the step of mounting the stent over the expandable stent delivery member.
In a presently preferred embodiment, the step of applying the plastic material comprises dissolving the plastic material in a solvent to form a dispersion, and applying the dispersion of the plastic material to the expandable stent delivery member. In a presently preferred aspect, the expandable stent delivery member is folded prior to the step of applying the dispersion of the plastic material on the expandable stent delivery member. After the dispersion is applied to the expandable stent delivery member, the solvent is preferably evaporated from the dispersion of the plastic material on the expandable stent delivery member to deposit the plastic material on the expandable stent delivery member. In another alternate approach, the step of applying the plastic material comprises dissolving the plastic material in a solvent to form a dispersion, mounting the stent on the expandable stent delivery member, and applying the dispersion of the plastic material on the expandable stent delivery member and the stent to form a thin coating of the dispersion of the plastic material on the expandable stent delivery member and the stent. The dilution of the dispersion of the dissolved plastic material in the solvent can be adjusted to control the thickness of the coating that is deposited, and preferably the dilution of the dispersion is adjusted to be sufficiently thin so as to not significantly interfere with subsequent inflation of the expandable stent delivery member and deployment of the stent. Alternatively, the plastic material may be applied by applying the plastic material by plasma grafting of the plastic material on the expandable stent delivery member. The plastic material also may alternatively be applied by plasma polymerization and deposition of the plastic material on the expandable stent delivery member. In one presently preferred embodiment, the plastic material is selected to adhere or fuse to the expandable stent delivery member. Where the plastic material fuses to the expandable stent delivery member, the plastic material preferably has a lower melt temperature than the expandable stent delivery member. The surface of the expandable stent delivery member may also be etched prior to applying the plastic material to the expandable stent delivery member. In another presently preferred aspect, the thickness of the applied plastic material does not exceed the thickness of the stent.
In an alternate preferred embodiment, the invention provides for a method for retaining a stent on an expandable stent delivery member, the stent having a surface defining a plurality of openings in the stent, wherein the method comprises the steps of mounting the stent over the expandable stent delivery member, and applying a relatively thick solution or dispersion of the plastic material that is insoluble in blood on the expandable stent delivery member through the openings of the stent adjacent to the stent struts. The plastic material on the expandable stent delivery member is then permitted to dry or set, to deposit the plastic material on the expandable stent delivery member, to retain the stent on the expandable stent delivery member. In this embodiment, in addition to the plastic materials mentioned above, the plastic material may also be a two part epoxy resin adhesive, a UV cured plastic material, or the like. In a presently preferred aspect, the step of applying the plastic material comprises forming knobs or fillets on the expandable stent delivery member adjacent to the struts of the stent, and preferably the thickness of the knobs or fillets does not exceed the thickness of the stent. The knobs or fillets serve to retain the stent on the expandable stent delivery member until the stent is deployed by expansion of the expandable stent delivery member, and when the expandable stent delivery member is deflated, after the stent is deployed, the knobs or fillets remain attached to the expandable stent delivery member.
In an alternate preferred embodiment, a stent may be retained on an expandable stent delivery member by mounting the stent over the expandable stent delivery member, melting a plastic material that is insoluble in blood, applying the melted plastic material onto the expandable stent delivery member through the openings of the stent adjacent to the stent struts, and allowing the plastic material to cool, to retain the stent on the expandable stent delivery member. In a presently preferred aspect, the applied melted plastic material forms knobs or fillets on the expandable stent delivery member adjacent to the openings of the stent, and preferably the thickness of the knobs or fillets does not exceed the thickness of the stent. It is desirable that the plastic material have a lower melt temperature than the expandable stent delivery member, in order to avoid damaging the stent delivery member. The plastic material may be of the same class or type of material of which the expandable stent delivery member is made, but having a lower melting temperature, allowing a good bond to be obtained without damaging the expandable stent delivery member. The knobs or fillets serve to retain the stent on the expandable stent delivery member until the stent is deployed by expansion of the expandable stent delivery member, and when the expandable stent delivery member is deflated, after the stent is deployed, the knobs or fillets remain attached to the expandable stent delivery member.
The plastic material may be chosen from plastics, organic materials, resins, two part resins, polymers, and copolymers, and combinations thereof. In a presently preferred embodiment, the plastic material is chosen from EAA copolymers, PVC, ethylene vinyl acetate copolymer (EVA), ethylene glycol butyl ether acetate (EBA) and ethylene methyl acrylate (EMA) ethylene acrylic ester copolymers, ethylene acrylic ester-maleic anhydride terpolymers, and acid copolymer resins.
In another presently preferred embodiment, the present invention provides for a system for applying a melted plastic material that is insoluble in blood on an expandable stent delivery member for retaining a stent on the expandable stent delivery member. The system comprises a holder for positioning the expandable stent delivery member with the stent disposed over the expandable stent delivery member, a microscope system for providing guidance for application of the melted plastic material on the expandable stent delivery member through the openings in the stent, a reservoir for the melted plastic material, and a high pressure controlled piston system connected in fluid communication with the reservoir for receiving the melted plastic material. The high pressure controlled piston system includes a nozzle, and means are provided for controlling the position and motion of the nozzle for dispensing the melted plastic material onto the expandable stent delivery member through the openings of the stent adjacent to the struts of the stent to retain the stent on the expandable stent delivery member.